Fermi observations of Cassiopeia and Cepheus: gamma-ray diffuse emission in the outer Galaxy

TL;DR

This study uses Fermi LAT data to analyze gamma-ray emission in the outer Galaxy, revealing insights into cosmic-ray density gradients and the CO-to-H2 conversion factor across different spiral arms.

Contribution

It provides new constraints on cosmic-ray distribution and molecular gas conversion factors in the outer Galaxy using high-resolution gamma-ray observations.

Findings

Improved understanding of cosmic-ray density gradient beyond the solar circle.

Enhanced constraints on the CO-to-H2 conversion factor in the outer Galaxy.

Detailed gamma-ray emission mapping of the second Galactic quadrant.

Abstract

We have used measurements by the Fermi Large Area Telescope (LAT) to study the interstellar gamma-ray emission in a region of the second Galactic quadrant, at 100 deg < l < 145 deg and -15 deg < b < 30 deg. This region encompasses the prominent Gould-Belt clouds of Cassiopeia, Cepheus, and the Polaris flare, as well as large atomic and molecular complexes at larger distances in several spiral arms. The good kinematic separation in velocity between the local, Perseus, and outer arms, and the presence of massive complexes in each, make this region very well suited to probe the gamma-ray emission from the interstellar medium beyond the solar circle. The unprecedented sensitivity and angular resolution of the LAT provide improved constraints on the gradient of the cosmic-ray densities and on the increase of the CO-to-H2 conversion factor, Xco, in the outer Galaxy.